How to apply a loop cut on a specific area on a big object?
I would recommend selecting all the faces of the object you want to put details on and hitting "P" to separate it as another object. 
This will help you with 2 things:
1) It will allow you to duplicate this object across the scene
2) And it will keep your large mesh clean and separate from the small details. 

Ideally, you want your scene to have no unnecessary loop cuts or geometry.

Select faces, Hit P



Now you can create a loop cut for your small object, but there is also a new object in your collection.